There is much interest in using biochemical or molecular biology techniques to produce therapeutic proteins with novel or enhanced properties. One desirable property is increasing biological activity, in particular increased circulating half-life of the protein.
Several methods have been employed to increase the biological activity of therapeutic proteins. These methods often focus on increasing the size of the therapeutic agents. One method of increasing a protein's size is through chemical cross-linking with another protein. For example, to increase the antigenicity of a protein, chemical cross-linking agents are used to conjugate the immunogenic protein to a carrier molecule such as immunoglobulin or serum albumin.
However, the conjugation of chemical compounds or inert molecules to a protein often results in a significant decrease of the overall biological activity of the protein, e.g., due to the conformational changes that occur, or due to increased steric hindrance as a result of the modification (Knusli, C., et al., Brit. J. Haematol., 82:654-663 (1992)).
Alternatives, such as peptide linkers have also been used. For example, U.S. Pat. No. 5,073,627 describes the use of a peptide linker to join a Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF) protein molecule to an Interleukin-3 (IL-3) protein molecule to form a fusion protein. Conventional peptide linkers, however, can be rigid and inflexible. As a result, the linked protein often cannot "flex " into the desired biologically active conformation exhibited by the wild type protein, or the cross-linker or carrier protein sterically hinders biological activity. Thus, there is still a need for linkers suitable to link proteins in such a manner as to increase biological activity, such as increasing in vivo half life.